I have a pair of Sennheiser HD4.50 headphones also. These headphones use the older Bluetooth version 4.0 and your set and mine, a Q60 require version 5.0. I purchased a Elegiant BTI-066 TV receiver/transmitter. The Elegiant pairs but does not connect. The Elegiant device shows on the Bluetooth speaker list when I am in the TV settings, but not the actual headphones.
I can't figure if the Elegiant should pair and connect or the actual HD 4.50 headphones should show on the speaker list and then pair/connect.